1. Field of the Invention
The present invention relates to a spacer grid supporting fuel rods of a nuclear fuel assembly, and more particularly, to spacer grid assembly equipped with a mixing vane mixing coolant that flows around fuel rods.
2. Description of the Related Art
A nuclear reactor is a device in which nuclear chain reactions of nuclear fission materials are artificially controlled to achieve several purposes such as generating heat, producing radioisotopes and plutonium, or forming radiation fields.
Generally, a light water reactor uses uranium enriched to about 2-5% Uranium-235. The uranium is fabricated as a cylindrical pellet weighing about 5 g for to process as nuclear fuel used in a nuclear reactor.
The pellets are bundled by hundreds together and inserted into a Zircaloy cladding tube in a vacuum, and an upper end cap is welded after insertion of spring and helium gas into the tube to manufacture a fuel rod. Finally, the fuel rod is burned through nuclear reaction in the nuclear reactor by forming a nuclear fuel assembly.
The nuclear fuel assembly and their elements are shown in FIGS. 1 to 3.
FIG. 1 is a schematic diagram illustrating a general nuclear fuel assembly, FIG. 2 is a plan view illustrating a spacer grid seen from above, and FIG. 3 is an exploded perspective view illustrating the spacer grid in detail.
Referring to FIG. 1, the nuclear fuel assembly includes a body formed with an upper end fitting 1, a lower end fitting 5, a guide tube 2 and a spacer grid 4, and a fuel rod 3 supported by a spring 6 (see FIG. 3) and a dimple 7 (see FIG. 3), inserted and formed inside the spacer grid 4.
To prevent scratch on the surface of the fuel rod 3 and damage to the spring 6 inside the spacer grid in the event of assembling a fuel assembly, the surface of the fuel rod 3 is painted with lacquer and the fuel rod 3 is inserted into a skeletal structure, then, the upper and lower end fittings 1 and 5 are attached and fixed thereto, finally, the assembling of the nuclear fuel assembly is completed. Then, after the assembly, a manufacturing process of the assembly is completed by checking intervals between fuel rods, distortion, total length, size, etc., after removal of the lacquer from the completed assembly.
Referring to FIGS. 2 and 3, the spacer grid 4 is formed in a grid shape by mutually engaging slots (not shown) on which each strip (a thin metal plate) is formed at regular intervals to divide space portions for insertion of each of a plurality of fuel rods 3.
The spacer grid 4 is arranged in number of 10 to 13 up and down and welded to the guide tube 2 having a length of 4 m. The spring 6 and the dimple 7 are regularly formed in each space portion that is divided by the spacer grid 4. The spring 6 and the dimple 7 are contacted to the fuel rod 3 (see FIG. 1) to maintain the interval of the fuel rods 3 (see FIG. 1), arrange the fuel rods 3 at determined positions and fix the fuel rod 3 due to elasticity of the spring 6.
Moreover, the spacer grid 4 is equipped with a plurality of mixing vanes 8 (see FIGS. 2 and 3) that integrated to the strips inside, protruded to downstream of coolant flow and bent for surrounding the fuel rod 3 to promote heat transfer caused by mixing coolant around the fuel rod 3.
Recently, development of nuclear fuel has been focused on high combustion efficiency and no defect. One of important obstacles that interfere with high-combustion efficiency is the phenomenon that is commonly referred to as DNB (departure from nuclear boiling). The DNB is affected by interval of nuclear fuel rods, pressure of a nuclear reactor system, heat flux, and coolant enthalpy and coolant velocity.
When the DNB is generated, a gaseous film is formed on the surface of the fuel rod in a corresponding portion. Then, due to decrease of heat transfer, the temperature of the fuel rods rapidly increases. If the state is maintained, finally, the fuel rods are damaged.
Accordingly, the nuclear reactor must be operated at a heat flux level lower than that at which the DNB occurs. This margin is referred to as “thermal margin”.
The nuclear reactor normally has regions of special-higher neutron flux or power density than other regions. One of factors that can cause the regions is a control rod channel for insertion of a control rod. When the control rod is withdrawn from the control rod channel, the space is filled with coolant that is a moderator. Thus, it increases efficiency of local moderating capacity and thereby increases power generated by adjacent fuel rods.
These regions having the high power density are hot channels having a higher rate of enthalpy than other channels. The hot channels limit the maximum operation conditions for the nuclear reactor and amount of power, since the channels reach the critical thermal margin first in comparison with other channels.
Conventional nuclear fuels have attempted to reduce variation of power density inside the nuclear reactor and improve the performance related to the DNB by providing the mixing vanes 8 integrated to the spacer grid 4, changing coolant flow and mixing coolant, thereby increasing heat transfer between the fuel rod 3 and the coolant.
The performance of the mixing vanes is affected by the size, shape, bend angle and position of the mixing vanes. The mixing vanes are so effective in regions of the hot channels in positions of fuel rods adjacent to positions of guide tubes for insertion of a control rod.
The conventional mixing vane has used the uniform coolant mixing vane pattern in the entire region of one spacer grid, or used a mixing vane pattern to be the same half image of the spacer grid as that of the rest half that is reflected on a mirror.
In a portion of the spacer grid coolant flow induced vibration is generated and thus it can cause fretting-wear of the fuel rod.
Hydraulic power generated by the mixing vanes is not balanced around the center of the spacer grid and thus it vibrates the spacer grid.
Accordingly, it has requested the spacer grid for improving performance related to the DNB and reducing vibration generated by the mixing vanes. In the U.S. Pat. No. 6,526,116 filed on Jul. 2, 1997, entitled “Nuclear Fuel Assembly with Hydraulically Balanced Mixing Vanes”, as a part of the improvement, it was suggested a spacer grid to divide the entire region of the spacer grid regularly around the center of spacer grid at uniform angles and to have the uniform mixing vane pattern in each section. In addition, the hydraulically balanced spacer grid is formed by enabling the mixing vane pattern to be formed in a shape rotatable around the center of the spacer grid at uniform angle, thereby enabling the mixing vane pattern to be symmetrical around the center of the spacer grid.
However, as shown in FIG. 4 corresponding to FIG. 2 of U.S. Pat. No. 6,526,116, in case of a central shape of the spacer grid in the exemplary embodiment wherein the mixing vane pattern is applied to the spacer grid having an array of 17×17, it can balance hydraulic moment generated by the mixing vane in regions except for a central portion of the spacer grid. But, because the pattern of the mixing vane that is the most adjacent to the center of the spacer grid rotates in the same direction around the center of the spacer grid, hydraulic moment deflected to the center of the spacer grid is generated. Accordingly, it is difficult to maintain hydraulic balance of mixing vane patterns of all spacer grids perfectly.
FIG. 5 illustrates a feature of the central portion of the spacer grid of FIG. 2 wherein the same mixing vane pattern as same as described above is applied to the spacer grid having an array of 16×16. FIG. 5 shows a pattern wherein the hydraulic moment is unbalanced by mixing vanes since the mixing vane adjacent to the center of the spacer grid is arranged in a form to rotate in the same direction around the center of the spacer grid.